Existing wire guidance systems typically use a pair of pickup coils which are excited by low frequency current in the guidance wire. The output of the sensing system is a pair of voltages, proportional to the distances from each coil to the wire. If both sensing coils are at equal distances from the wire (and the coils are mounted symmetrically on the vehicle) then the output voltages will be equal - indicating that the wire is located under the center of the vehicle. However, if the vehicle moves to one side of the wire, the opposite coil will put out a greater voltage, which can be used to turn the vehicle front wheels in the proper direction.
Typically, the coils are mounted ahead of the wheels often in a non-magnetic bumper. This approach suffers from the problem that electromagnetic noise often induces false signals and distorts the location indication of the system. A sudden move to one side or the other, especially as the result of a bump or lurch, of the vehicle can also degrade the reliability of the sensing. Thus, the vehicle guidance is affected. This is particularly true when the vehicle speed is considerable, e.g. 60 mph or more.
A need, therefore, exists for a guidance sensor which is reliable and is not effected by electromagnetic or other type of noise and which is responsive to sudden moves of the vehicle travelling at relatively high speeds.